Longitudinal Plasmon Resonance Band Research Articles

Exploring the photophysical interaction of xanthene dyes with gold nanorods by optical spectroscopic techniques and in-vitro cytotoxicity studies of dye-nano conjugates

The current study investigates the photophysical interactions of xanthene dyes such as fluorescein (FL) and rose bengal (RB) dyes with cetyltrimethylammonium bromide stabilized gold nanorods (CTAB-AuNRs) using optical spectral techniques, and computational interaction analyses. When xanthene dyes are conjugated with CTAB-AuNRs, significant modifications in their optical spectroscopic characteristics are found. In the absorption spectrum analyses of dye-AuNRs systems, the appearance of longitudinal plasmon resonance bands and the hypochromic behavior of xanthene dyes were observed in response to the interaction of CTAB-AuNRs. The gathered extinction spectral results indicated that there were significant electrostatic interactions between the positively charged CTAB-AuNRs and the negatively charged xanthene dye molecules. At the same time, the substantial quenching with a reduced quantum yield of the dye molecules is shown in the emission spectra of xanthene dyes with CTAB-AuNRs. The strong electrostatic interactions and ground state complex between xanthene dyes and CTAB-AuNRs have been suggested as the mechanism of emission quenching. The results of the emission quenching are supported by the calculated high binding constant values. Additionally, the results of computational interaction investigations also demonstrate the near closeness of xanthene dye molecules and CTAB-AuNRs. Finally, the cytotoxicity of free dyes and dye-nanoconjugates was next tested against breast (MCF-7) and lung (A-549) cancer cells. All the tested samples exhibit significant antiproliferative activity against MCF-7 and A-549 cancer cells. These cytotoxic study results showed that cells treated with xanthene dyes were less sensitive than FL-AuNRs and RB-AuNRs dye nanoconjugates.

PH and near-infrared active; chitosan-coated halloysite nanotubes loaded with curcumin-Au hybrid nanoparticles for cancer drug delivery

In this work, we propose biofriendly in-situ preparation method of Au NPs (hexagonal and rod-shape structures) in the lumen as well as the surface cage of biocompatible halloysite nanotubes (HNTs) using curcumin (CUR) as anticancer drug and subsequently coating with bio-adhesive chitosan (CS) as a polysaccharide. The formation of Au NPs and their interactions with CUR and CS exist in the HNTs has been characterized by FTIR, XRD, XPS, STEM techniques. Interestingly, Au NPs showed longitudinal plasmon resonance bands at 760 and 980 nm that indicate the near-infrared (NIR) responsive property of hybrid nanoparticles. Rod shape and hexagonal structures of Au NPs were produced as confirmed by TEM images. The loading efficiency of CUR was found as much as 12%. Importantly, more CUR release was achieved under acidic conditions (pH 5.5) than basic conditions (pH 7.4). The anticancer potential of HNT hybrid nanoparticles on MCF-7 cancer cells was studied and showed efficient anticancer activity under intracellular tumor cell environment (pH 5.5) than extracellular conditions (pH 7.4). Moreover, the developed HNT hybrid nanoparticles consisting of Au NPs (NIR responsive property) and pH-responsive CUR release could make it suitable for cancer cell-targeted drug delivery platform with NIR-imaging.

Seedless co-surfactant-based dimensional and optical tunability of gold nanorods with simultaneous pH regulation

Intense electromagnetic fields generated by the gold nanorods (GNRs) owing to their strong plasmon resonance make them promising candidate for applications in cancer diagnostics, photothermal therapy, single-molecule detection, drug delivery, etc. Use of GNRs in these applications as plasmon resonance probes depends on the extent to which their plasmon resonance band can be tuned into NIR region. In this study, we propose a facile seedless approach to synthesize GNRs in a cetyltrimethyammonium bromide (CTAB)–benzyledimethylhexadecylammonium chloride (BDAC) co-surfactant system with simultaneous pH regulation that yields GNRs with tunable plasmon resonance in tissue transparent NIR region. The role of binary co-surfactants, pH of the growth solution, and influence of the volume of the reducing agent on GNRs growth has been studied. Under optimized conditions, the longitudinal plasmon resonance band of GNRs can be tuned from 780–1300 nm by regulating the pH of the growth solution from 2.5 to 1.3. The role of co-surfactants and pH in the regulation of dimensions and thus plasmonic properties of GNRs synthesized by single-step seedless approach has been evaluated.

Assembled Behavior of Noble Metal Nanoparticles Related to Capping Agents in Sol–Gel Process.

The assembly of noble metal nanoparticles (NPs) created using different capping agents was investigated in a sol–gel process. By controlling the hydrolysis and condensation of tetraethyl orthosilicate (TEOS), noble metal NPs were assembled and fixed permanently through the anisotropic growth of SiO₂ shell. Various morphological noble metal NP assemblies including single bead, dimer, and pearl-chain were fabricated by controlling the addition of TEOS. Noble metal NPs were homogenously coated with SiO₂ shell without using a bulk polymer or silane coupling agent as the surface primer. It is worth noting that the assembled noble metal NPs which stabilized by different stabilizers performed different UV-vis adsorption spectra. With increasing the length of assemblies, longitudinal Plasmon resonance band appears and can be tuned from visible to near-infrared. These assemblies could be applies in biomedicine area, particularly as biosensors, Raman-tag particles, and photothermal therapy.

One-pot high-yield synthesis of single-crystalline gold nanorods using glycerol as a low-cost and eco-friendly reducing agent

The formation of gold nanorods (AuNRs) has recently attracted great attention due to their shape-dependent optical properties that are important for many applications. The development of simpler and safer methods for the high-yield synthesis of AuNRs employing low-cost and easily handled reagents is thus of great importance. Here, we introduce, for the first time, a one-pot seedless method for the preparation of single-crystalline AuNRs in almost 100 % yield based on the use of glycerol in alkaline medium as an eco-friendly, low-cost and pH-tunable reducing agent. The synthesized AuNRs were characterized by UV–Vis–NIR spectroscopy, FEG–SEM and HRTEM. The effect of the presence of capping agent (CTAB) and the concentration of reactants (glycerol, NaOH and AgNO3) on the yield and aspect ratio (AR) of AuNRs is discussed. The AR and yield of AuNRs showed a clear dependence on the pH and temperature of the reaction mixture as well as on the concentration of AgNO3 added as an auxiliary reagent. The longitudinal plasmon resonance band of the resulting AuNRs can be tuned between 620 and 1200 nm by varying the reaction conditions. AuNRs with an aspect ratio (AR) of around 4 were obtained in almost 100 % yield at room temperature and under mild reducing environment. The formation of AuNRs is faster at higher pH (>11) and higher temperature (>30 °C), but the AuNR yield is smaller (<70 %). Variation in the pH of the reaction mixture in the range 12–13.5 results in the formation of AuNRs with different ARs and in different yields (27–99 %). Detailed study of the AuNRs crystallography by HRTEM showed that the AuNRs grow in [001] direction and have a perfect single-crystalline fcc structure, free from structural faults or dislocations. The present green method, which introduces glycerol as a tunable reducing agent with a pH-dependent reducing power, can provide a more general strategy for the preparation of a wide range of metallic nanoparticles.

Fast coagulation of gold sols. the formation of interparticle contacts at early stages

The fast coagulation of gold sols with concentrations of 46-nm particles equal to (0.8–30) × 1010 cm–3 initiated by an indifferent electrolyte has been studied by spectrophotometry. Variations arising in two absorption bands upon the formation of asymmetric clusters in the sols of plasmonic nanoparticles have been analyzed. Analysis of the kinetic data on variations in the high-energy mode, the intensity of which is governed by the contributions of both individual nanoparticles and their aggregates, has shown that, at early stages, the coagulation has the character of a bimolecular reaction. Extinction spectra of dimers, in which gold nanoparticles are located at different distances from each other, have been calculated with the use of the generalized Mie theory. A “plasmon ruler,” which represents the shift of the longitudinal plasmon resonance band of dimers relative to that of individual particles as depending on the distances between the surfaces of the particles in the dimer, has been constructed based on the calculated data. Using the plasmon ruler and experimental data, the distance between nanoparticles in dimers formed during coagulation has, for the first time, been determined. It has been found that, at the initial stages of gold sol fast coagulation, when the process develops as a bimolecular reaction, nanoparticles that have formed dimers are not in the direct contact.

Various Au Nanoparticle Organizations Fabricated through SiO2 Monomer Induced Self-Assembly

A novel method has been developed to fabricate the assembly of Au colloidal nanoparticles (NPs) using SiO(2) monomers. The key strategy was the use of a controlled sol-gel procedure including hydrolysis, deposition, and condensation of tetraethyl orthosilicate (TEOS). Namely, the assembly of Au NPs was created by the anisotropic deposition of SiO(2) monomers and subsequent permanent fixing by the growth of a SiO(2) shell. Various assemblies of Au NPs such as dimer, trimer, and pearl-chain morphology were fabricated by systematically changing the concentration and injection speed of TEOS. A longitudinal plasmon resonance band was observed as a result of the assembly of Au NPs and can be tuned from visible to near-infrared by altering the length of pearl-chain morphology. In addition, single Au NP was homogeneously coated with a SiO(2) shell by means of controlling the deposition rate of SiO(2) monomers during a Stöber synthesis without the use of a silane coupling agent or bulk polymer as the surface primer to render the Au surface vitreophilic. The Au NPs (mean size 11.4 nm in diameter) were thus encapsulated into SiO(2) beads with a wide range of sizes (from 20 to 50 nm in diameter). These pure SiO(2)-coated Au beads with tunable shell thickness should be crucial for biosensors, particularly as Raman-tag particles.

Evaluation of electromagnetic enhancement of surface enhanced hyper Raman scattering using plasmonic properties of binary active sites in single Ag nanoaggregates

We found a binary active site of surface enhanced Raman scattering (SERS) and surface enhanced hyper Raman scattering (SEHRS) in single Ag nanoaggregates by single particle spectroscopy. The intensity fluctuation of SEHRS was canceled by dividing SEHRS intensity by SERS intensity on the basis of the binary active site analysis. Thanks to the identification of the plasmons common to both SERS and SEHRS we revealed that an enormous enhancement in SEHRS is ascribed to the electromagnetic (EM) enhancement entirely coupled with longitudinal plasmon modes excluding other kinds of enhancement factors. Our results indicate that EM enhancement factors of metal nanostructures are estimated from spectral information on the longitudinal plasmon resonance band obtained by the scattering or absorption spectra of the nanostructures.

Water-Soluble Conjugated Polymer-Induced Self-Assembly of Gold Nanoparticles and Its Application to SERS

Gold nanochains were prepared by the assembly of citrate-stabilized gold nanospheres induced by cationic conjugated polymers. This assembly method was rapid, and the assembled product was very stable. A longitudinal plasmon resonance band was formed as a result of the coupling of gold nanoparticles and can be tuned from visible to near-infrared by adjusting the polymer/Au molar ratio. The gold nanochains were used as a SERS substrate and gave an enhancement factor of 8.4 x 10 (9), which is approximately 400 times larger than that on the isolated gold nanosphere substrate. The giant SERS enhancement is ascribed to the large electromagnetic fields of coupled gold nanoparticles.

Surfactant-Directed Synthesis and Optical Properties of One-Dimensional Plasmonic Metallic Nanostructures

AbstractOne-dimensional metallic nanostructures such as nanorods and nanowires are of tremendous interest for electronic, sensing, and catalytic applications. Shape anisotropy introduces new optical properties in gold and silver nanoparticles, such as longitudinal plasmon resonance bands in the visible and near-IR portion of the spectrum. Different approaches employed for the shape-controlled synthesis of silver and gold nanocrystals include chemical, electrochemical, and physical methods. The chemical route for the synthesis of nanorods and nanowires, in which metal salts are reduced in an aqueous solution, is one of the most widely used methods. This route commonly employs a surfactant as the directing agent to introduce asymmetry in the nanocrystal shape. Variation in the concentration of precursor salt and the surfactant, the nature of the surfactant, the nature and concentration of reducing agents, the presence of external salts, and the pH of the reaction solution all affect nanocrystal shape, dimension, and yield. The size and shape of the nanocrystals affect the position of the plasmon bands, which in turn has been widely used in surface-enhanced spectroscopies that include both Raman and fluorescence. The aqueous, surfactant-directed route also promises the synthesis of more complex nanostructures with additional desirable properties.

Starch-Mediated Shape-Selective Synthesis of Au Nanoparticles with Tunable Longitudinal Plasmon Resonance

We report the synthesis ofAu nanoparticles, with tunable longitudinal plasmon band and shape selectivity, mediated by starch in the presence of ultrasonic waves. The synthesis was carried out by reduction of HAuCl4, at various concentrations, using H2O2 as the reducing agent. When the reactions were carried out in the absence of ultrasonic waves, there was no occurrence of the longitudinal resonance band, while the transverse plasmon resonance band shifted toward a higher wavelength. Transmission electron microscopic measurements revealed an increase in particle sizes with increasing higher initial HAuCl4 concentration. On the other hand, in the presence of ultrasonic waves, as the initial concentration of HAuCl4 was increased, while the transverse plasmon resonance band remained the same, the longitudinal plasmon resonance band increasingly shifted toward a higher wavelength. Transmission electron microscopic measurements revealed the change in shape from spherical to triangular to hexagonal particles with increasing initial HAuC14 concentration. We also report that the starch-stabilized nanoparticles could be precipitated from the solution by a starch digesting enzyme which also binds with the particles resulting in its precipitation.